For many years soil moisture content has been measured by a device, known as a tensiometer which measures the surface tension of the water in a capillary state. A typical tensionmeter is disclosed in U.S. Pat. No. 2,878,671. Tensiometers include a tubular porous ceramic sensor or bulb closed at one end and sealed at its open end to a tube which is usually provided at its upper end with a transparent sight tube closed by a sealing cap. The tube is connected to a vacuum gauge or mercury manometer. In use, the tensiometer is filled with water and the sensor is placed in a carefully prepared opening in the ground, so as to obtain a snug contact between the sensor and the soil. Often two or three tensiometers are placed with their sensors at different depths. When the soil is saturated, all pore space filled with water, the vacuum gauge will indicate essentially atmospheric pressure which is zero gauge, or 760 mm Hg absolute pressure. As the soil moisture is reduced, the capillary dimensioned water particules tend to move into the pores of both the sensor and the soil; however their surface tension will resist and produce a subatmospheric pressure in the tensiometer indicator on the gauge. As the soil moisture lowers, the moisture seals will progressively open and permit some air to enter the tensiometer. Once this occurs, the expansible character of air renders the tensiometer inaccurate, hence, the provision of a sight tube to detect air. Hence, the provision of a cap for removal to replace air with water.
A plant watering device utilizing a ceramic sensor similar to the tensiometer sensor which has met with success is shown in U.S. Pat. No. 3,758,987. In this case, the porosity is such that air is permitted to pass through the sensor at a sub-atmospheric pressure to partially relieve a vacuum created in a sealed water reservoir. However, the flow of air from the soil through the sensor wall tends to carry minute matter which tends to cloq the sensor. When the soil becomes saturated, as during a heavy rain or excessive irrigation, water is drawn through the sensor, thereby blocking the flow of air through the small tubing connected to the sensor and rendering the device inoperative.